Conventionally, a receiver unit includes various filters whose cut-off frequency must be very precisely fixed. However, by reason of temperature variations or of the variations, for example, in the power supply voltage, the parameters of the filters used exhibit very wide variations typically between 40% and 50%.
In order to limit these variations, the filter cut-off frequency is controlled by a control signal delivered by another system called a master system. This conventional filter structure is described, for example, in the article ‘Integrated Continuous-Time Filter Design—An Overview’, Yannis Tsividis, IEEE Journal of Solid-State Circuits, vol. 29, no. 3, Mar. 1997, pp. 166-176. The master system may include for example, a phase-locked loop operating according to a stable reference frequency delivered by an external component such as, for example, a quartz oscillator. The master system includes linear amplifiers that are identical to (or possibly homothetic with) those incorporated into the filter, and with an amplification factor that is internally controlled by the same signal used for controlling the cut-off frequency of the filter.
This control of the master/slave type (the slave being the filter) typically allows a precision of around 5%. Furthermore, in order to make the filter operate over various frequency ranges, a divider or a multiplier by N may be used between the control signal output from the master system and the control signal of the filter, if the relationship between the amplification factor of the amplifiers incorporated in the filter and its control quantity is perfectly proportional. Thus, the ratio N may be introduced between the amplification factor of the master system amplifiers and the amplification factor of the amplifiers of the filter.
Owing to the relationship between the amplification factor and the frequency (detailed herein below), the cut-off frequency of the latter is varied as a function of the oscillation frequency of the master system, in the same ratio N. Accordingly, if V denotes the control signal output from the master system, k is the amplification factor of the ideal amplifiers used in the filter and the master system, and by the relationship k=A*V, where A is the coefficient linking the control signal, then Equation 1 below results.
                              f          osc                =                                                            A                ·                V                ·                k                                            2                ⁢                π                ⁢                                                                  ⁢                C                                      ⁢                                                  ⁢            and            ⁢                                                  ⁢            fc                    =                                    A              ·              V              ·              k                                                      N                ·                2                            ⁢              π              ⁢                                                          ⁢              C                                                          (                  Eqn          .                                          ⁢          1                )            
In Equation 1, fosc is the master system oscillation frequency, fc the filter cut-off frequency and C a reference capacitance value for the device in question (master system or filter to be controlled). Using these relationships, Equation 2 results.fc=fosc/N  (Eqn. 2)
In other words, by controlling and adjusting the amplification factor k of the amplifiers incorporated into the filter by means of a control signal such as that described hereinabove, a ratio of N is established between the amplification factors of the master system and of the filter, and hence the oscillation frequency of the master system and the cut-off frequency of the filter.
However, the relationship of proportionality between the value of the control signal of the amplifier and its actual amplification factor is never perfect. For example, non-linearities in the control of the amplifier always exist and thus lead to the impossibility of using directly the principle stated hereinabove between the theoretical relationship of proportionality and that observed between the oscillation frequency of the master system and the cut-off frequency of the filter, in this example.
There is therefore a need for a device that is capable of establishing a ration N between two amplification factors, and more particularly, a device capable of establishing two frequencies which are independent of the non-linearity phenomena in the control of the amplifiers.